In known valve-regulated lead-acid (VRLA) batteries, each positive electrode, sometimes called a positive "plate," includes a grid or foil sandwiched between an electrochemically active paste. A number of positive electrodes are alternately interleafed with a number of negative electrodes, sometimes called the negative "plate," with each electrode of one polarity separated from the adjacent electrode of opposite polarity by a porous separator material, typically a glass microfiber mat. The cell is completed by adding electrolyte and subjecting it to a formation charging process that activates it. The entire apparatus is typically contained within a suitable plastic case or other container.
The interface between the grid or foil and the paste is known as the corrosion layer. While all of the chemistry/electrochemistry that takes place here is not well understood, the establishment of a strong, well-defined corrosion layer is felt to be necessary for long cycle life in lead-acid batteries. With some grid/foil alloys, in particular pure lead, lead/calcium and lead/low tin compositions, there is not sufficient corrosion to establish a strong layer and in place of this a so-called "passivation" layer is created by alkaline oxidation of the grid/foil surface. This corrosion/passivation layer is composed primarily of PbO which is protected from neutralization by the sulfuric acid electrolyte by a layer of mixed lead compounds, but primarily lead sulfate. It acts as a perm-selective membrane that allows the underlying PbO to exist in an alkaline environment. This corrosion/passivation layer can act as an electrical insulator or at least reduce the conductivity between the grid/foil and the active material paste and thus can have a dramatic impact on the electrochemical properties of the cell. The corrosion/passivation layer appears to play an integral role in at least two important characteristics of cell performance: self-discharge and cycle life.
The term "self discharge" refers to a series of different chemical reactions within the cell that can reduce the storage time, or shelf life, due to consumption of electrolyte. The open-circuit voltage directly reflects the specific gravity, or concentration of electrolyte within the cell, and it is also linearly proportional to discharge capacity. Any self-discharge reaction that consumes electrolyte reduces both storage time and discharge capacity. Corrosion of the positive grid/foil on open-circuit stand is one mode of self discharge and does consume electrolyte. The term "cycle life" refers to the number of usable cycles of discharge and recharge available from the cell. The cycle life figure is dependent upon a number of conditions under which it is determined, as well as the basic cell construction. For example, a cell which achieves 80% of its initial amp-hour rating after 500 cycles and reaches 50% after 1,000 cycles will have two different "cycle life" values, depending upon the criterion used for termination (80 or 50% of initial capacity). Another measure, related to cycle life, is total usable capacity. This term refers to the sum of the cycles over the life of the cell multiplied by the amp-hours at each cycle. It can also be expressed as the area under the curve produced by a plot of data showing discharge capacity (in amp-hours) versus cycle number.
It can be appreciated that it is desirable for cells to have low self-discharge--or voltage decay--rates. Low self-discharge rates allow for longer storage times without complete loss of capacity. It can also be appreciated that it is desirable for cells to have long cycle life to allow many discharges and recharges before the cell is replaced. It is similarly important that the total usable capacity be high, thereby implying that the amp-hour capacity of the cell is reasonably constant over the bulk of the cycle life. The total usable capacity represents the amount of useful work a cell can provide.
It can also be appreciated from the foregoing and common knowledge within the industry that establishment of a strong corrosion layer during manufacture, formation and cycling will result in a cell with long cycle life. It is also known that grid/foil alloys that produce a strong corrosion layer will be susceptible to ongoing corrosion that will reduce storage time. Conversely, it is known that grid/foil alloys that do not have significant corrosion properties will result in the creation of a passivation layer in the positive plate between the grid/foil and the positive active material. This passivation layer will tend to protect the grid/foil from corrosion but, as mentioned, it will act to inhibit the passage of current during charging and thus can, when severe, result in drastically short cycle life--a phenomenon termed premature capacity loss, or PCL. It should be appreciated that all of the foregoing comments apply only to the positive plate in a lead-acid cell and not the negative plate.
Generally, both the grid/foil and the active material include lead in various compositions along with smaller quantities of other materials. In particular, tin and tin compounds have been used in lead-acid electrochemical cells. For example, U.S. Pat. No. 5,368,961 discloses a cell having a grid alloy of about 2.5% tin. The use of tin in previous cells has generally been confined to having it in the grid/foil metallic alloy, as opposed to including some form of tin in the paste.
It has been found that the inclusion of small percentages of tin in the grid/foil allows some control over the nature of the corrosion/passivation layer that is formed, and thus a corresponding control over the self-discharge and cycle life characteristics of the cell. This is apparently due to the fact that when the grid/foil surface has areas that are relatively high in tin content, either in the grain boundary areas or within the grains themselves, corrosion results in tin, probably in the form of soluble tin(II) or insoluble SnO.sub.2, migrating into the corrosion/passivation layer. This tin acts as a conductor to ameliorate the insulative effects of the passivation layer and to thereby enhance the conductivity, and thus the current flow, between the grid/foil and the positive active material. At tin percentages at or below about 1.1-1.3%, true alloys are formed and the tin distribution is relatively uniform. Above about 1.3%, the solubility of tin in lead is exceeded and the grain boundaries and the grains contain relatively high concentrations of tin. In the former case, passivation layers tend to form and dominate cell performance due to the low amount of corrosion of the grid/foil and, hence, low levels of tin compounds in the passivation layer. When the tin content of the grid/foil metal exceeds the 1.3% level, corrosion of the high tin areas takes place and a true corrosion layer forms, with any passivation areas being sporadic or non-existent and, where formed, containing significant levels of tin compounds. From the foregoing discussion, it should be appreciated that these two conditions involve trade-offs between good self-discharge characteristics and good cycling performance. It should also be appreciated that for tin contents of up to about 1.5% the layer between the grid/foil and the positive active material will be some combination of corrosion and passivation structures, and not necessarily exclusively one or the other.
For example, grids/foils having 2-3% tin levels (all percentage figures being by weight) have certain desirable performance characteristics, in comparison with grids/foils such as pure lead or those having about 0.5% tin or less. In particular, a cell with a positive grid/foil with 2-3% tin provides very good cycling performance, i.e., the cell is able to produce many discharge/recharge cycles with good capacity and a high total usable amp-hours. However, such a cell has an undesirably high self-discharge rate such that the storage time is unacceptable for most commercial applications. It is believed that this high self-discharge rate is due to corrosion of the high tin areas and the grain boundaries, thus consuming significant quantities of the sulfuric acid electrolyte and thereby increasing the rate of decrease of the open-circuit voltage.
Cells with grids/foils having about 1% tin or less exhibit much improved self-discharge characteristics, but do not provide the excellent cycle life of cells having grids/foils with 2-3% tin. Because they are true alloys, it is believed that the foils with 1% tin or less have a much reduced corrosion rate compared to the foils with 23% tin. It is believed that this is due to the tin distribution being more or less homogeneous in the true alloys, with no areas of high tin concentrations in the bodies of the grains and a low level of tin enrichment in the grain boundaries (relative to 2-3% tin foils). The 1% tin (or less) foils thus reduce the self-discharge levels of the cells due to limited tin corrosion, but because of this they do not release sufficient tin into the corrosion/passivation layer to improve markedly the cycling performance.
It would thus be desirable to perfect a cell with good self-discharge characteristics as well as good cycle life characteristics. Clearly, a low corrosion rate of the positive grid/foil is necessary for a low self-discharge rate and, due to it being a long-term failure mode, long cycle life. Conversely, a positive foil with a relatively high corrosion rate apparently creates a corrosion layer that is very favorable for long cycle life, but the corresponding self-discharge rate is high and deterioration of the foil will eventually result in cycle life failure. Clearly, the tin content of the positive grid/foil is an important variable with respect to these two sets of characteristics. Moreover, the experiments in altering the tin content of the positive grid/foil suggest that the design of a lead-acid cell involves a compromise, or tradeoff, of those characteristics. It is believed that no commercial lead-acid cell has previously been designed with both a very low self-discharge rate and a very high cycle life.
Japanese patent publication 1979-49538 published Apr. 18, 1979 describes a "ready-to-use lead-acid battery pole plate" in which SnSO.sub.4 or SnO was added to "non-activated paste material." This publication is directed toward a system for reducing the storage deterioration in certain kinds of cells. Such deterioration, according to the publication, is due to the production of non-conductive lead oxide between the pole(grid) and the active material. This is now a well-understood phenomenon called "storage passivation" that occurs in the processing of so-called "dry charged" plates. The publication asserts that such deterioration is diminished by the addition of SnSO.sub.4 or SnO of less than 400 mesh to the active material. The mechanism and chemistry of the asserted effect is not specified. Moreover, the asserted advantages based on the experimental results set forth in the publication are marginal at best. For example, the publication indicates that the voltage of a prior art test cell was 1.55V when subjected to a high-rate discharge while the voltages of the cells in accordance with the asserted invention were 1.57 to 1.59V. Similarly, the "self-sustaining period" until the voltage dropped to 1.00V was 3 min. 54 sec. for the prior art test cells and 4 min. 01 sec. to 4 min. 05 sec. for the cells in accordance with the asserted invention. This represents an improvement of only a few percent, which is probably well within the margin of error of the experiment.
The experimental results in the Japanese publication were also less than dramatic for tests conducted after a 10 month storage period. The prior art test cell showed an initial voltage on high-current load of 1.23V and a self-sustaining period until 1.00V of 3 min. 25 sec., while the cells in accordance with the asserted invention showed an initial load voltage of 1.27 to 1.50V and a self-sustaining period until 1.00V of 3 min. 35 sec. to 3 min. 47 sec.
The Japanese publication teaches that the addition of SnSO.sub.4 or SnO is to occur prior to the addition of sulfuric acid:
SnSO.sub.4 of varied particle sizes (200-400 meshes and under 400 meshes) was added to the lead powder in the ratio of 0.5-3% and was fully mixed before adding sulfuric acid.
The introduction of SnSO.sub.4 or SnO prior to the introduction of sulfuric acid appears not to produce an effective paste, for reasons that are not fully understood. The very marginal improvement realized in the experiments set forth in this Japanese publication appear to support that conclusion. In any event, the Japanese publication apparently never issued as a patent in Japan, and no corresponding United States patent has been located. Further, the publication teaches nothing about cycling characteristics or self-discharge characteristics. Finally, the present invention does not depend upon a particular mesh size of SnSO.sub.4 or SnO, as proper paste mixing and the resultant electrochemical effects are more effective when the tin salt used is dissolved during mixing.
There have been many prior art methods with the goal of improving cycle performance other than by altering the tin content of the positive grid/foil. Such methods have included, for example, increasing the cell plate stack compression, using high-density active materials, using thick paste layers and using conductive glass fibers in the positive active material. However, these prior-art methods include disadvantages such as difficulties in manufacturing and processing and high material costs.